The rational design of helium bonds

The chemistry of helium has hitherto been confined to experimental and theoretical analysis of small molecules containing three to five atoms in the gas phase. Here a new suggestion is made for compounds of helium deriving from a recent proposal that five-coordinate carbon might be captured as a frozen S(N)2 transition state. A series of logical steps, originally discussed as postings and comments to two blogs, led to the outcome described here of a central hypervalent atom bound on one face by a small cyclic carbon ligand, with the other free face having an interaction to a helium atom with the topological properties of a charge-shift rather than a covalent bond. Although high-level theory predicts these helium bonds to be quite short with relatively high stretching frequencies, the kinetic barriers to the loss of helium are predicted to be small, and are not increased by the strategy of having bulky substituents on the ring ligand.